With these substantial losses, the Islamic State faces a threat to its legitimacy, especially since this militant group principally seeks to establish a caliphate. This distinguishes it from other terrorist organizations. With this threat to its credibility, the Islamic State has proceeded to target Europe. The attacks on Paris and Brussels serve as a counterbalance to the group’s cessation of territory within the Middle East, moving the theater of conflict to their enemies home turf. Europe is a particularly attractive target, given its porous borders and lack of Muslim integration within European communities.

While the attack on Brussels serves as a reminder that continued military success of aerial campaigns against the Islamic State may not be entirely sufficient to stop future terrorist attacks in the West, it also raises concerns about the United States’ grounds for sustained military action against the group. Although President Obama has requested the legislative branch’s attention in this matter and steps have been taken by Senate Majority Leader Mitch McConnell to put legislation authored by Lindsey Graham on the floor, Congress has yet to issue a new authorization for the use of military force (AUMF) with respect to the Islamic State.

Although the President has asserted that the 2001 AUMF, which authorized the use of force against “Al-Qaeda and associated forces” in the wake of the terrorist attacks on September 11, 2001, grants him authority to utilize force against the Islamic State, the President’s actions would be further legitimized by a new authorization, as it would validate his actions politically through Congress’ express sanctioning. The general populace serves as the ultimate limit on both the executive and legislative branches. Successful sustainment of the nation’s military campaigns relies on the continued support of the public. However, the President can assert that his actions are legally permissible, as unilateral action would not necessarily be illegitimate and without prior historical precedent.

Unilateral action by the Executive Branch is consistent with the historical expansion of the executive branch’s constitutional war powers. The era after World War II can be characterized as an epoch during which presidential war powers were particularly expansionary; historical precedent including President Truman’s commitment of forces in Korea, President Eisenhower’s request for authorization of engagement of American forces in the Middle East, and President Kennedy’s order for the quarantine of Cuba all serve as illustrations of expansion of war powers authority within the executive branch. These examples, among others, involved the unilateral action of the president. Even within the context of Eisenhower’s request for congressional authorization, the President asserted that the actions would be “inherent in the authority of the Commander in Chief,” noting however that “a suitable congressional resolution would clearly and publicly establish the authority of the President.”

There have been a number of prior instances in which the president has asserted his authority on the basis of prior Congressional authorizations and treaties. While these instruments and pieces of legislation were often authored in a crisis or within a context distinct from when they were actually used, these treaties and authorizations have proven to be particularly useful tools for the president in asserting his authority to utilize force.

Although the President may have a legal basis for the United States’ sustained involvement in a military campaign against the Islamic State absent a new congressional authorization, there are considerable benefits to be gained by an AUMF tailored specifically to conflict with the Islamic State. Congressional action and debate may serve as an important opportunity for the American public to be educated on the contours of this conflict. Furthermore, coordination and cooperation between the legislative and executive branches may serve a legitimizing function, signaling unity and resolve within the United States, and further validating the country’s actions as a whole through the political process.

Terrorist attacks such as the one in Brussels serve as a reminder that while our aerial campaigns against the Islamic State in Iraq and Syria may be successful, the Islamic State is not precluded from attacks outside of these territories. Sustained military involvement by the United States will only be legitimized by the cooperation and coordination of our coordinate branches, especially as the conflict against this group continues to shift and evolve.